1 CHAPTER 11 Data Link Control Solutions to Review Questions and Exercises Review Questions 1. The two main functions of the data link layer are data link control and media access control . Data link control deals with the design and procedures for commu-nication between two adjacent nodes: node-to-node communication. Media access control deals with procedures for sharing the link. 2. The data link layer needs to pack bits into frames . Framing divides a message into smaller entities to make flow and error control more manageable. 3. In a byte-oriented protocol , data to be carried are 8-bit characters from a coding system. Character-oriented protocols were popular when only text was exchanged by the data link layers. In a bit-oriented protocol , the data section of a frame is a sequence of bits. Bit-oriented protocols are more popular today because we need to send text, graphic, audio, and video which can be better represented by a bit pat-tern than a sequence of characters. 4. Character-oriented protocols use byte-stuffing to be able to carry an 8-bit pattern that is the same as the flag. Byte-stuffing adds an extra character to the data section of the frame to escape the flag-like pattern. Bit-oriented protocols use bit-stuffing to be able to carry patterns similar to the flag. Bit-stuffing adds an extra bit to the data section of the frame whenever a sequence of bits is similar to the flag. 5. Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment. Error control refers to a set of procedures used to detect and correct errors. 6. In this chapter, we discussed two protocols for noiseless channels: the Simplest and the Stop-and-Wait . 7. In this chapter, we discussed three protocols for noisy channels: the Stop-and-Wait ARQ , the Go-Back-N ARQ , and the Selective-Repeat ARQ . 8. Go-Back-N ARQ is more efficient than Stop-and-Wait ARQ. The second uses pipelining , the first does not. In the first, we need to wait for an acknowledgment for each frame before sending the next one. In the second we can send several frames before receiving an acknowledgment.

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2 9. In the Go-Back-N ARQ Protocol , we can send several frames before receiving acknowledgments. If a frame is lost or damaged, all outstanding frames sent before that frame are resent. In the Selective- Repeat ARQ protocol we avoid unnecessary transmission by sending only the frames that are corrupted or missing. Both Go-Back-N and Selective-Repeat Protocols use sliding windows . In Go-Back-N ARQ, if m is the number of bits for the sequence number, then the size of the send win-dow must be at most 2 m − 1 ; the size of the receiver window is always 1. In Selec-tive-Repeat ARQ, the size of the sender and receiver window must be at most 2 m − 1 . 10.

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